1. Field of the Invention
The present invention relates to a process for preparing sintered titanium nitride (TiN) cermets. More particularly, the present invention relates to a process for preparing sintered titanium nitride cermets, wherein thermal decomposition of titanium nitride is prevented due to the remarkably improved wettability of the particle surfaces of TiN powder with respect to the nickel liquid phase.
2. Description of the Prior Art
Generally, titanium nitride has a high hardness and good heat resistance as well as a golden luster. Titanium nitride is mainly used in the field of ornamental materials and cutting tools. Although titanium nitride has characteristics that are useful for ornamental materials and cutting tools, its physical properties are exhibited when it is coated onto other materials because of some difficulties in its preparation.
Since the melting point of titanium nitride is 2927.degree. C., its fabrication is almost impossible by conventional material processing methods. The only available method is to prepare titanium nitride powders and produce sintered masses therefrom.
The high melting point, however, requires a very high temperature to consolidate titanium nitride powders. Like other nitrides, titanium nitride decomposes thermally when it is sintered at a high temperature. Accordingly, a sintering method that enables the lowering of the sintering temperature is a key to a better process for producing sintered titanium nitride cermets.
Sintering aids which accelerate densification are required to sinter titanium nitride powders at a relatively low temperature. There are very few publications dealing with the sintering of titanium nitride powders. Mitani and Fulmhara obtained a sintered density greater than 93% of the theoretical value. See H. Mitani, H. Nagai and M. Fukuhara, Journal of the Japan Institute of Metals, 42, 582 (1978); M. Fukuhara and H. Mitani, Journal of the Japan Institute of Metals, 43, 169 (1979); M. Fukuhara and H. Mitani, Trans. JIM, 21,211 (1980); and M. Fukuhara and H. Mitani, Journal of the Japan Society of Powder and Powder Metallurgy, 26, 143 (1979). Mitani and Fukuhara employed a liquid phase sintering method wherein nickel powder is added as a sintering aid and sintering is performed at a temperature above 1353.degree. C. (1626 K.) where the nickel liquid phase appears.
However, a disadvantage in the above method is that thermal decomposition of titanium nitride is not prevented during sintering. Thermal decomposition results in the formation of pores due to the nitrogen gas that is produced during thermal decomposition and results in a sintered mass with incomplete densification.
The occurrence of residual pores during the sintering of titanium nitride results because the liquid phase of nickel does not completely cover the titanium nitride particles at the sintering temperature. As a result of continued research to overcome the above disadvantages, the present invention was completed by means of preventing thermal decomposition of titanium nitride by remarkably improving its wettability to the nickel liquid phase.